How did Macduff know King Duncan?

Macduff was a noble Scotsman. Duncan I [d. August 14, 1040] was the King of Scotland. Therefore, Macduff knew who King Duncan was. He was the King's subject. Additionally, Macduff appeared to number among those noble Scotsmen upon whom the King counted and with whom he moved most closely. Specifically, Macduff was the nobleman whom the King had asked to make sure that he was up and moving on to the next stop after the Macbeths' castle at Inverness. Macduff didn't get to carry out that task, because the King and his two royal guards were stabbed to death the night before.